Progressive lenses are usually used by persons who need different lens power for different distances. Conventional progressive lenses usually designed in agreement with the fact that usually a wearer lowers his gaze in order to look at close distances. The design of conventional progressive lenses is usually based on two main assumptions: it is assumed that the main usage of the lens is along its vertical central meridian, with possible deviation to account for small eye convergence for near vision tasks, and it is assumed that the objects' distances distribution is symmetrical with respect to central vertical meridian of the visual field.
These assumptions make it possible, and indeed that is what usually being done, to make the lens design for one eye (left or right) as the “mirror reflection” of the lens design for the other eye (for the same prescription) in terms of its power and astigmatism distributions across the lens.
However, the conventional design may result in primarily vertical or almost vertical power distribution. Sometimes, the power distribution of the lens is matched to specific vertical object distribution of wearer's visual field. In this conventional approach the areas on the sides of the lens usually regarded as disturbing distortion areas, caused by power progression along vertical central meridian of the lens, in which lens designers try by various methods to minimize the distortion.
As a result of the conventional design described above, the lens may limit the wearer's horizontal natural eyes movement and forces him to use his head to look at the sides. This in turn may result in a lens being suitable for only horizontally symmetrical visual tasks.